WASHINGTON (AP) — Special counsel Robert Mueller is warning everyone in President Donald Trump's orbit: If they lie about contacts between his election campaign and Russians, they could end up on the wrong end of federal criminal charges.

With the disclosure of the first criminal cases in his investigation, Mueller also has shown that he will not hesitate to bring charges against people who were close to the campaign even if the allegations don't specifically pertain to Russian election interference and possible collusion with the Trump camp.

Court papers unsealed Monday revealed an indictment against Trump's former campaign chairman, Paul Manafort, and a guilty plea by another adviser, who admitted to lying to the FBI about meetings with Russian intermediaries.

Perhaps more unsettling for the White House, the plea by George Papadopoulos came weeks ago, and his initial arrest has been kept quiet for months, all while he has been cooperating with federal agents. The charges had been sealed specifically to keep the news of his guilty plea from discouraging others from cooperating with the special counsel or from destroying evidence.

At Papadopoulos' plea hearing earlier this month, one of Mueller's prosecutors, Aaron Zelinsky, hinted at the possibility of more charges to come. The Mueller probe is "a large-scale ongoing investigation of which this case is a small part," Zelinsky said, according to a transcript.

The developments, including the unexpected unsealing of the guilty plea, usher Mueller's investigation into a new, more serious phase. And the revelations in the plea about an adviser's Russian contacts could complicate the president's assertions that his campaign had never coordinated with the Russian government to tip the 2016 presidential election in his favor, the central issue behind Mueller's mandate.

On Tuesday morning, Trump sought to distance himself from Papadopoulos. "Few people knew the young, low level volunteer named George, who has already proven to be a liar," the president tweeted.

The Kremlin denied Russia is implicated by the first criminal cases against Trump associates.

Government spokesman Dmitry Peskov says the connections between Papadopoulos and a man he believed to have links to the Russian Foreign Ministry did not prove any complicity by the Russian government.

"So far Russia doesn't figure in any way in these charges which have been made," he said. Peskov added that accusations of Russian meddling in the election remain "unfounded."

Manafort, who steered Trump's campaign for much of last year, and his business associate Rick Gates are under house arrest on charges that they funneled payments through foreign companies and bank accounts as part of their political work in Ukraine.

Papadopoulos, also a former campaign adviser, faces further questioning and then sentencing in the first — and so far only — criminal case that links the Trump election effort to Russia.

Manafort and Gates, who pleaded not guilty in federal court, are not charged with any wrongdoing as part of the Trump campaign, and the president emphasized that immediately. He said on Twitter that the alleged crimes occurred "years ago," and he insisted anew there was "NO COLLUSION" between his campaign and Russia.

However, Papadopoulos admitted he was told in April 2016 by a London-based professor that the Russians had "dirt" on Democrat Clinton in the form of "thousands of emails," well before it became public that the Democratic National Committee and Clinton campaign chairman John Podesta's emails had been hacked.

The court papers do not name the professor. But a comparison of conversations cited in the papers and emails previously obtained by The Associated Press show the professor is Joseph Mifsud, honorary director of the London Academy of Diplomacy. Mifsud did not immediately respond to a request for comment.

The court filings don't say whom Papadopoulos may have told about the Russian claims about possessing emails damaging to Clinton. According to a previous filing in the case, Papadopoulos told the FBI that he didn't tell anyone in the Trump campaign about the dirt on Clinton because he didn't know if it "was real or fake."

Papadopoulos was not charged with having improper communications with Russians but rather with lying to FBI agents when asked about the contacts, suggesting that Mueller — who was appointed in May to lead the Justice Department's investigation — is prepared to indict for false statements even if the underlying conduct he uncovers might not necessarily be criminal.

Mueller's investigation has already shadowed the administration for months, with investigators reaching into the White House to demand access to documents and interviews with key current and former officials.

The Papadopoulos plea occurred on Oct. 5 but was not unsealed until Monday. He admitted lying to FBI agents about the nature of his interactions with "foreign nationals" who he thought had close connections to senior Russian government officials.

One of the foreign nationals is described in court papers as a Russian with connections to the country's foreign ministry. The emails obtained by the AP show he is Ivan Timofeev, director of programs at the Russian International Affairs Council in Moscow. Timofeev did not answer calls seeking comment Tuesday.

The emails also show Papadopoulos discussing his attempts to line up a meeting between Trump and the Russian government and forwarding Manafort correspondence he had with Timofeev and Mifsud. Manafort and Gates rebuffed such a meeting or a foreign trip for Trump in email discussions.

Papadopoulos has been cooperating with investigators, according to the court papers. His lawyers hinted strongly in a statement Monday that their client has more testimony to provide.

There, too, the White House scrambled to contain the potential fallout, with press secretary Sarah Huckabee Sanders contending that "any actions that he took would have been on his own."

The criminal case against Manafort, who surrendered to the FBI in the morning, had long been expected.

The indictment naming him and Gates, who also had a role in the campaign, lays out 12 counts including conspiracy against the United States, conspiracy to launder money, acting as an unregistered foreign agent, making false statements and several charges related to failing to report foreign bank and financial accounts. The indictment alleges the men moved money through hidden bank accounts in Cyprus, St. Vincent and the Grenadines and the Seychelles.

In total, more than $75 million flowed through the offshore accounts, according to the indictment. Manafort is accused of laundering more than $18 million.

Outside the courthouse, Manafort attorney Kevin Downing attacked the charges and said "there is no evidence that Mr. Manafort or the Trump campaign colluded with the Russian government."

Manafort's indictment doesn't reference the Trump campaign or make any allegations about coordination between Russia and campaign aides. But it does allege a criminal conspiracy was continuing through February of this year, after Trump had taken office.

Manafort, 68, was fired as Trump's campaign chairman in August 2016 after word surfaced that he had orchestrated a covert lobbying operation on behalf of pro-Russian interests in Ukraine. The indictment against Manafort and Gates says the pair had managed a covert Washington lobbying operation on behalf of Ukraine's ruling political party.

Gates directed the work of two prominent Washington lobbying firms, Mercury LLC and the Podesta Group. The indictment doesn't refer to the companies by name, but the fallout at one was swift.

Prominent Washington lobbyist Tony Podesta, a Democrat and brother to Clinton campaign official John, resigned Monday, seeking to avoid further enmeshing his firm in the controversy, according to a person familiar with the decision who spoke anonymously to preserve relationships with former colleagues.

President Donald Trump listens during a meeting on tax policy with business leaders in the Roosevelt Room of the White House, Tuesday, Oct. 31, 2017, in Washington. (AP Photo/Evan Vucci)

A court artist drawing shows President Donald Trump's former campaign chairman, Paul Manafort, center standing and Manafort's business associate, Rick Gates, in federal court in Washington, Monday, Oct. 30, 2017, before U.S. Magistrate Judge Deborah A. Robinson. Seated at front left is Manafort's attorney Kevin Downing. Manafort and Gates have pleaded not guilty following their arrest on charges related to conspiracy against the United States and other felonies. The charges are the first from the special counsel investigating possible coordination between the Trump campaign and Russia. (Dana Verkouteren via AP)

Paul Manafort, left, leaves Federal District Court in Washington, Monday, Oct. 30, 2017. Manafort, President Donald Trump's former campaign chairman, and Manafort's business associate Rick Gates pleaded not guilty to felony charges of conspiracy against the United States and other counts. (AP Photo/Alex Brandon)

Rick Gates, right, leaves federal court in Washington, Monday, Oct. 30, 2017. Paul Manafort, President Donald Trump's former campaign chairman, and Manafort's business associate Gates pleaded not guilty to felony charges of conspiracy against the United States and other counts. (AP Photo/Susan Walsh)

Paul Manafort walks from Federal District Court in Washington, Monday, Oct. 30, 2017. Manafort, President Donald Trump's former campaign chairman, and Manafort's business associate Rick Gates pleaded not guilty to felony charges of conspiracy against the United States and other counts. (AP Photo/Alex Brandon)

The car believed to be transporting President Donald Trump's former campaign chairman, Paul Manafort, prepares to leave the FBI Field Office in Washington, Monday, Oct. 30, 2017. Manafort, and a former business associate, Rick Gates, have been told to surrender to federal authorities Monday, according to reports and a person familiar with the matter. (AP Photo/Andrew Harnik)

In this photo July 21, 2016, photo, then-Trump campaign manager Paul Manafort, right, watches with then-Republican presidential candidate Donald Trump and Manafort's chief deputy Rick Gates, left, as Ivanka Trump rehearses for the Republican National Convention in Cleveland. President Donald Trump's former campaign chairman, Manafort, and Gates, a former Manafort business associate, have been indicted on Oct. 30, 2017, on charges of conspiracy against the United States and other counts. (AP Photo/Evan Vucci)

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